The following discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge as at the priority date of the application.
Environmentally responsible disposal of waste tyres presents a tremendous challenge. Approximately 20 million waste tyres are generated in Australia annually and the number approximates 1000 million on a global scale. Traditionally, the majority of tyres were buried however many industrialised countries including Australia are enforcing bans to prohibit whole tyres going to landfill.
Most commercial processes dealing with waste tyres involve tyre shredding as a preliminary step to break tyres into managable pieces. Unfortunately, the very characteristics that make tyres long lasting and safe on the road such as durability, resistance to puncture and slicing, and resistance to decomposition at moderate temperatures, combine to make tyres exceptionally difficult to disintegrate. Commercially available tyre disintegrators include slicing machines, hammer mills, pyrogenic crushing, debeaders and manglers. While shredding overcomes some process difficulties for handling a bulky material of varying size, it greatly increases the capital and energy requirements of these processes.
Existing art teaches that shredding of waste tyres may be followed by physical processes to remove steel and fibre reinforcing materials to recover the rubber as various grades of rubber chips or crumb. Other methods dealing with waste tyres have been adopted by the cement industry, where tyres are used as a source of fuel in high temperature kilns where all of the solid residues are incorporated in the cement product.
An alternative process has been developed by Molectra, a company in Queensland, Australia, described in the specification for Australian patent AU2000278910B, which is a process for recovering materials from tyres that involves soaking tyres in oil and a volatile solvent followed by microwave heating.
A number of pyrolytic techniques to process waste tyres have also been described. However, these techniques have been beset with difficulties. Primary barriers to successful pyrolysis operations are both economic and technical.
With existing methods for the recovery of materials from waste tyres, the capital costs are high and the products from pyrolysis do not have sufficient value or purity to compete with virgin materials. Reaction products are chiefly liquid and gas hydrocarbons, carbon black and steel and in the existing processes, the products suffer from cross contamination to varying degrees, preventing their acceptance as raw materials.
In existing pyrolysis methods, heat is transferred to the rubber either by conduction, convection, infra red or microwave radiation. As rubber is a poor conductor of heat, these methods can lead to large temperature variations throughout the decomposing material. Variations in heating rates, temperature and vapour residence times during decomposition, lead to a vast spectrum of reaction mechanisms and associated hydrocarbon products.
Previously described processes utilise a low oxygen environment and may additionally use a range of materials to contact and transfer heat to the decomposing polymer products. United States patent specification US2002/10072644 A1 describes waste tyres being immersed in a bath of molten aluminium at a temperature of 800° C. in a refractory lined vessel to decompose the rubber to mostly gas products and carbon. Previously published processes also describe use of low melting point metals such as tin and lead as well as molten salt baths, alkaline earth metals, sand or gravel beds and mineral substances such as granulated silicates or aluminosilicates.
A shortcoming of existing pyrolysis methods is contamination of the carbon and steel solid residues. Pyrolysis char is highly porous and consists of many interconnected pores, cracks and channels formed as the volatile components escape from the rubber-carbon matrix. Granular materials and molten salts infiltrate these pores and are difficult to remove from the char. Molten lead and other metals may also penetrate the char and cause unacceptable levels of contamination.
One benefit of using a liquid metal heat transfer medium is that the high specific heat of metals and high conductance allows energy to be rapidly transferred to the waste polymer resulting in a rapid rate of decomposition at a more controlled temperature. High temperature molten metals have also been described for destroying toxic waste. In these cases the object is to break up the organic compounds at high temperatures in the order of 800-900 C into smaller organic gaseous species that do not have the toxic characteristics of the original compounds.
Lowering the decomposition temperature for rubber polymers results in longer chain molecules being formed, which favours production of liquid hydrocarbons, which on further refinement, can be used as transport fuels. The condensed oil from tyre pyrolysis may contain hydrocarbons with boiling point fractions in the range including kerosene, petrol and diesel. However, transport fuels must meet stringent quality and performance criteria, and particularly sulphur levels. Typically the condensable hydrocarbons have an offensive pungent odour due to high concentrations of complex sulphur compounds derived from the sulphur which is added to rubber in the vulcanization process.
Following the phasing out of lead in fuel, sulphur is being reduced from levels of over 500 ppm a few years ago to mandatory standards of 10 ppm. Low sulphur fuels allow catalytic converters with high efficiency to be used on vehicles. Like lead, sulphur poisons catalytic conversion processes of emissions reduction.
Sulphur is used to cross-link polymer chains in the process of vulcanization. In general terms, vulcanization results in about one in every 200 carbon double bonds forming a cross-link site with between 3 and 12 sulphur atoms. Sulphur at levels in tyres of about 2 phr (parts per 100 rubber) leads to crude decomposition hydrocarbons containing sulphur at levels far exceeding the limits for transport fuel standards.
A range of sulphur crosslinked polymer compounds constitute the rubber contained in waste tires. Decomposition involves the breaking of carbon—carbon and sulphur—carbon bonds. The presence of reactive sulphur atoms leads to the formation of thiophene derivatives responsible for the offensive odour of pyrolysis oil. These and other sulphur related compounds produce and detectable odour and detract from the commercial value of tyre derived products when present at levels above about 20 ppm. Hydrogen sulphide is also a by product contained in the non condensable gasses, which must be dealt with in order to meet acceptable environmental operation parameters.
While sulphur can be removed from crude oil fractions by hydro-treating and the same process could be applied to pyrolysis oil from waste tyres, this would require a high capital investment and extensive processing.
This invention seeks to provide a process and apparatus suitable for use in recycling rubber tyres, and recovering useful products from them. It is an object, in a preferred form of the invention, to provide apparatus for recycling whole tyres, without the need for shredding or cutting the tyres into pieces.
Throughout the specification unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.
Throughout the specification unless the context requires otherwise, the word “include” or variations such as “includes” or “including”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.